This invention relates to wireless digital communication.
With ownership of home computers increasing, many households now have more than one computer. Purchasing separate peripherals (printers, scanners, modems, removable storage media) for each computer can be expensive and wasteful, when often only one of each peripheral may be needed. Also, households typically provide only one phone line for use by computer modems, requiring users of multiple computers to take turns accessing the on-line services. A home-based local area network (LAN) would allow the computers to share these peripherals, but most inexpensive networking hardware currently available requires cabling to physically connect the computers. As multiple-home-computer users often keep their computers in separate rooms, use of cable-based networking requires running cables through the house walls. This, in addition to the complications of administering network routers or hubs, has discouraged home use of current networking technology.
Some LAN technologies do not depend on re-wiring homes. Some use existing wiring, such as power lines or phone lines. To date, these technologies can experience interference and security problems (for example, power lines are typically shared with other nearby homes). In addition, existing wired LANs typically do not permit mobile access to the network by portable laptop computers. Wireless LANs (WLANs) have also been pursued for networking personal computers. For example, infrared-based networks have been developed, particularly in work-place environments for communication with mobile personal digital assistants (PDAs).
Digital radio communication has also been explored for WLANs. In 1985, the FCC established regulations to allow unlicensed use of certain bands if spread-spectrum (SS) techniques are used. In SS transmission, the energy radiated during transmission is spread across a wide spectrum of frequencies (or xe2x80x9cchannelsxe2x80x9d) and is therefore less likely to cause substantial interference with other radio communications. FCC SS regulations allow greater transmission of power to be used without special licensing, increasing the attainable range of communications for unlicensed systems.
Two principal SS transmission techniques include direct-sequence (DS) and frequency-hopping (FH). In DSSS, spreading is achieved through multiplication of the data by a binary pseudo-random sequence whose chipping rate is many times the data rate. In FHSS, the carrier frequency remains at a given channel for a duration of data transmission, and then xe2x80x9chopsxe2x80x9d to a new channel elsewhere within the spreading bandwidth.
DSSS allows for coherent demodulation, in which the receiver exploits knowledge of the carrier""s phase reference to detect the encoded signals. With FHSS, however, phase coherence is difficult to maintain between hops. Non-coherent demodulation results in the advantage of reduced complexity, but at a cost of an increased probability of error. FHSS typically enables high data rates to be achieved without requiring the high-speed logic that an equivalent DSSS system would require. Also, a FHSS system can employ frequency diversity, which combats multipath fading by transmitting data at multiple frequencies, increasing the likelihood that data will be transmitted and received uncorrupted. The data requirements of digital communications equipment is increasing as the typical size of data files becomes larger. Complex modulation schemes have been used to handle higher bandwidth requirements and more efficiently use the bandwidth allocated to RF devices. However, existing modulation schemes can be either very logic-intensive or require very accurate and expensive clock references. An example of the former type of scheme is differential quadrature phase shift keying (DQPSK), in which complex (IQ) demodulation is required to recover the encoded data. An example of the latter type of scheme is frequency shift keying (FSK), in which small changes in frequency are used to represent the encoded data. The change in frequency in FSK is usually very small and the receiving system may have difficulty determining whether a particular shift in frequency it detects represents real data or an offset between transmitter and receiver reference clocks.
In general, in one aspect, the invention features a method for radiofrequency (RF) transmission of digital information including generating an RF signal using a voltage-controlled oscillator (VCO), stabilizing the RF signal from the VCO by providing an error signal from a phase-locked loop (PLL) to an input of the VCO, and combining the digital information with the error signal of the PLL input to the VCO, thereby causing variations in frequency of the RF signal from the VCO that represent the digital information.
Embodiments of the invention may include one or more of the following features. The RF signal can be broadcast. The error signal of the PLL can be provided to the VCO by a loop filter. The rate of change of the digital information can be faster than a response time of the PLL. The digital information can be encoded such that it has a duty cycle which is substantially constant over the response time of the PLL. The digital information can be encoded according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard. A channel frequency of the RF signal can be changed according to a series of channel frequencies, wherein the series of channel frequencies is determined by generating a series of at least a first and a second channel-select signal, each channel-select signal comprising a frequency-valid indicator and a frequency-specification indicator, sending each channel-select signal in turn to the PLL, and changing the tuning frequency of the PLL according to the frequency specification of the first channel-select signal upon receiving the frequency-valid indicator of the second channel-select signal. The number of distinct channel frequencies in the series of channel frequencies can be a prime number. The series of channel frequencies can be repeated upon completion, and no channel can be used more than once in each repetition of the series of channel frequencies. The transmitted RF signal can be received, demodulated, a signal level threshold can be determined from the demodulated RF signal, and the demodulated RF signal can be compared to the signal level threshold to regenerate the digital information from the demodulated RF signal. The RF signal output of the VCO can be used as a local oscillator to demodulate the received RF signal.
In general, in another aspect, the invention features a method for RF transmission of digital information including generating an RF signal using a voltage-controlled oscillator (VCO), stabilizing the RF signal from the VCO by providing an error signal from a phase-locked loop (PLL) to an input of the VCO, encoding the digital information such that it has a duty cycle which is substantially constant over the response time of the PLL, forming a data packet containing the encoded digital information, and combining the data packet with the error signal of the PLL input to the VCO, thereby causing variations in frequency in the RF signal from the VCO that represent the data packets.
Embodiments of the invention may include one or more of the following features. The data packet can have a duty cycle which is substantially constant over the response time of the PLL. The digital information can be encoded and the packets can be formed according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard.
In general, in another aspect, the invention features a method for receiving digital information sent by an RF transmitter using the IrDA 4PPM data encoding standard including receiving a transmitted RF signal, demodulating the RF signal, determining a signal level threshold from the demodulated RF signal, comparing the demodulated RF signal to the determined signal level threshold to thereby regenerate IrDA-formatted data from the demodulated RF signal, and reproducing the digital information by decoding the IrDA-formatted data.
In general, in another aspect, the invention features a method of implementing communication between a plurality of host computers using spread-spectrum RF transmission, including exchanging digital information between a first host computer and a first RF transceiver, encoding the digital information using pulse position modulation (PPM) according to the IrDA 4PPM data encoding standard, forming a data packet from the encoded digital information according to the IrDA 4PPM data encoding standard, generating an RF signal using a first voltage-controlled oscillator (VCO), stabilizing the RF signal from the first VCO by providing an error signal from a first phase-locked loop (PLL) to an input of the VCO, combining the data packet with the error signal of the first PLL input to the VCO, thereby causing variations in frequency in the RF signal from the first VCO that represent the data packet, broadcasting the RF signal to a second RF transceiver, demodulating the RF signal at the second RF transceiver, determining a signal level threshold from the demodulated RF signal, comparing the demodulated RF signal to the determined signal level threshold to thereby regenerate the data packets from the demodulated RF signal, reproducing the digital information by decoding the data packet, and exchanging the digital information with a second host computer.
Embodiments of the invention may include one or more of the following features. Media access control (MAC) functions can be placed in host computer software at both the first and second host computers. The possibility that multiple transceivers will transmit simultaneously can be reduced by each transceiver waiting a pseudorandom time from the end of the last transmission or reception before transmitting. The pseudorandom time can be determined by a pseudorandom number generator, by clock drift of one host relative to another, or by interrupts generated by other operations of the host computers.
In general, in another aspect, the invention features an apparatus for RF transmission of digital information including a VCO, the VCO arranged to generate an RF signal, a PLL, the frequency input of the PLL coupled to the RF signal output of the VCO, an encoder, the encoder arranged to convert the digital information into a form where it has a data rate faster than a response time of the PLL, and a coupler, the coupler coupling both the error signal output of the PLL and the encoded digital information to an input of the VCO.
Embodiments of the invention may include one or more of the following features. An antenna can be coupled to the RF signal output of the VCO. The coupler can be a loop filter. The encoder can be arranged to convert the digital information into a form where it has a duty cycle which is substantially constant over the response time of the PLL. The encoder can convert the digital information according to the pulse position modulation,(PPM) scheme defined by the IrDA 4PPM data encoding standard. A channel frequency selector can change a channel frequency of the RF signal according to a series of channel frequencies, wherein the selector determines the series of channel frequencies by the selector generating a series of at least a first and a second channel-select signal, each channel-select signal comprising a frequency-valid indicator and a frequency-specification indicator, the selector sending each channel-select signal in turn to the PLL, and the PLL changing its tuning frequency according to the frequency specification of the first channel-select signal upon receiving the frequency-valid indicator of the second channel-select, signal. The number of distinct channel frequencies used by the selector in the series of channel frequencies can be a prime number. The selector can repeat the series of channel frequencies upon completion, and not use any channel more than once in each repetition of the series of channel frequencies.
In general, in another aspect, the invention features an apparatus for RF reception of IrDA 4PPM-encoded digital information including an antenna, the antenna receiving a transmitted RF signal containing the IrDA 4PPM-encoded digital information, a low-noise amplifier, the low-noise amplifier amplifying and filtering the received RF signal, a local RF oscillator, the local RF oscillator configured to produce a frequency in proximity to the channel frequency of the received RF signal, a mixer, the inputs of the mixer coupled to the output of the low-noise amplifier and the local RF oscillator, an FM demodulator, the input of the FM demodulator coupled to the output of the mixer, and a decoder, the decoder extracting the digital information from the demodulated RF output of the FM demodulator according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard.
In general, in another aspect, the invention features an apparatus for RF transmission and reception of digital information including a VCO, the VCO arranged to generate an RF signal, a PLL, the frequency input of the PLL coupled to the RF signal output of the VCO, an encoder, the encoder arranged to convert the digital information into a form where it has a data rate faster than a response time of the PLL, a coupler, the coupler coupling both the error signal output of the PLL and the encoded digital information to an input of the VCO, a switch, the switch configured to couple its input to at least one of its outputs, a first antenna coupled to a first output of the switch, the first antenna arranged to transmit the RF signal, an antenna arranged to receive a transmitted RF signal, a low-noise amplifier coupled to the attenna, the low-noise amplifier amplifying and filtering the received RF signal, a mixer, one input of the mixer coupled to the output of the low-noise amplifier and another input of the mixer coupled to a second output of the switch, an FM demodulator, the input of the FM demodulator coupled to the output of the mixer, and a decoder, the decoder reproducing the digital information from the demodulated RF output of the FM demodulator.
Embodiments of the invention may include one or more of the following features. The antenna arranged to receive the transmited RF signal can be the same antenna as the first antenna, and the apparatus can include a power amplifier, the input of the power amplifier coupled to the first output of the first switch, and a second switch, the second switch configured to couple the first antenna either to the input of the low-noise amplifier for reception, or to the output of the power amplifier for transmission. The decoder can reproduce the digital information from the demodulated RF output of the FM demodulator according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard.
Advantages of the invention may include one or more of the following. A 2.4 Ghz transceiver can be coupled to a number of personal computers to form a robust, low cost RF WLAN. The RF WLAN can support relatively high-bandwidth digital signal communication between computers or other devices. The RF transceiver does not require generating both a carrier signal and an information bearing signal that are then mixed, requiring an additional (relatively expensive) mixer and filter. The approximately 2.4 Ghz signal transmitted by the transceiver is directly generated by a relatively simple VCO and PLL feedback circuit whereby digital information is directly injected into the feedback circuit. By employing a particular encoding scheme used in infrared communications to encode the digital information injected into the feedback circuit, frequency drifts that would otherwise be expected by such direct injection can be avoided. More than one transceiver can be coupled to the same computer to increase data throughput by splitting data packets among more than one communications channel. By placing most Medium Access Control (MAC) features in software instead of hardware, relatively cheap and available CPU cycles can be used to perform MAC operations instead of additional MAC circuitry in the transceiver.
Other features and advantages of the invention will become apparent from the following description and from the claims.